1. Field of Technology
The present invention relates to an image forming apparatus such as a photocopier and printer, particularly to an image forming apparatus that uses a sensor to detect the patch density formed on a recording sheet and to control the image forming conditions, and provides offset regulation control of shifting an image formation area in response to the offset during conveyance of the recording sheet.
2. Description of Related Art
One of the conventional apparatuses includes an image forming apparatus that forms an image by the steps of:
forming an electrostatic latent image on an image carrier in conformity to image data;
generating a developed image (toner image) by developing the electrostatic latent image using developer (toner);
transferring the generated toner image from the image carrier to a recording sheet (recording sheet) by the transfer current from a transfer device;
separating the recording sheet from the image carrier by the separation current from a separation device; and
using heat and pressure to fix the recording sheet carrying the toner image.
In such an image forming apparatus, an image of approximate density is obtained by controlling various forms of parameters under the image forming conditions. One of image forming conditions is the transfer condition. To be more specific, the transfer current is supplied to the image carrier and recording sheet from a transfer device and the toner image is transferred to the recording sheet. The percentage of the toner transferred to the recording sheet varies according to the value of the transfer current. The transfer efficiency is also known to vary in conformity to the size, thickness and material of the recording sheet, temperature, humidity, the amount of charge of toner on a photoreceptor, the amount of deposited toner, the contamination of the transfer device, the amount of water contained in the recording sheet, the degree of adhesion between the recording sheet and photoreceptor, the rotating speed of the photoreceptor, recording sheet conveyance speed and other factors. These various conditions must be taken into account before making adjustments. However, it is difficult to make adjustments to ensure the optimum transfer current value.
Thus, Japanese Unexamined Patent Application Publication No. H5-241390 to be described below discloses the technique wherein the toner image (patch) in conformity to the image data of a predetermined density is formed on the recording sheet, the density of the toner image on the recording sheet is measured, and the transfer current is corrected in conformity to the difference between the density estimated from the image data and the actually measured density.
For such a patch, the patch on the image carrier is read by a sensor using the non-transfer area on the image carrier, i.e., the space between the images to be transferred in the process of image formation, whereby the real-time implementation is possible in the process of image formation. This arrangement improves productivity without having to suspend image production. This type of technique is also disclosed in the Japanese Unexamined Patent Application Publication No. 2005-289035.
Further, the Japanese Unexamined Patent Application Publication No. 2007-316237 discloses the technique of automatically detecting the position for forming the above-mentioned patch and correcting the position for forming the patch.
Another apparatus in the conventional art is an image forming apparatus that provides offset regulation control in such a way that the offset in the direction perpendicular to the direction of the recording sheet being conveyed (the main scanning direction) is detected during recording sheet conveyance, and the image formation area on the image carrier is shifted in response to the offset. Thus, even if the recording sheet is conveyed by such an offset regulation control with the recording sheet being offset in the conveyance mode, an image will be transferred at a desired position.
Starting from the leading-edge of the image formation area for the recording sheet when an offset has occurred, the offset regulation control is performed continuously up to the leading edge of the image formation area of the recording sheet where a different offset has occurred, including the non-image formation area.
Still another apparatus in the conventional art is an image forming apparatus wherein, as described above, various forms of predetermined patches such as a density patch or registration mark are formed on the non-image area of the image carrier, and various forms of patches on this image carrier are read by a sensor, whereby the above-mentioned image forming conditions are controlled.
The inventors of the present application have found out in the above-mentioned case that, when the patch forming position is changed to the direction of main scanning by the aforementioned offset regulation control, the patch on the image carrier cannot be detected by the sensor if the patch is formed in finer patterns.
For example, in FIGS. 8 and 9, both the transfer area and the non-transfer area including the patch before the next transfer area starts are shifted to the right of the drawings by the offset regulation control on the Nth page.
In a similar manner, in FIGS. 8 and 9, both the transfer area and the non-transfer area including the patch are shifted to the left of the drawings by the offset regulation control on N+1th page until the next transfer area starts.
To put it another way, in FIG. 9, the transfer area and the non-transfer area immediately thereafter are in the same direction indicated by the broken line. This suggests that the same offset regulation control is performed in the transfer area and the non-transfer area immediately thereafter.
Since the shift in the transfer area agrees with the offset of the recording sheet, the relative misalignment is cancelled. Thus, an image free of misalignment is formed on the recording sheet. In the meantime, if the patch on the image carrier in the non-transfer area is shifted, positional misalignment occurs with respect to the sensor at the fixed position. This leads to the inability of reading a patch.
In view of the problems described above, it is an object of the present invention to provide an image forming apparatus capable of ensuring compatibility between the control of image forming conditions using a patch and offset regulation control conforming to the recording sheet offset.